Influenza is an acute, infectious respiratory system disease caused by influenza viruses. According to the difference in antigen of internal nucleoprotein (NP) and matrix protein (M), influenza viruses can be classified into influenza A, B and C viruses. Pandemic of influenza A virus can cause high morbidity and mortality and a serious threat to human health (W.H.O. 2003; Coleman 2007). In the twentieth century, influenza A viruses mainly caused three major flu, i.e. H1N1 in 1918, H2N2 in 1957, and H3N2 in 1968, and killed about 50 million people (Kilbourne 2006; Taubenberger, Hultin et al. 2007). Influenza A in 2009 was also caused by H1N1 influenza virus (Dawood, Jain et al. 2009; Zimmer and Burke 2009), which spread rapidly and attracted the attention of the world. According to statistics, about 300-500 thousands of people worldwide died annually from influenza (Fiore, Shay et al. 2007).
Up to now, FDA-approved anti-influenza drugs mainly include two categories. The first category includes Tamiflu (Oseltamivir) and Relenza (zanamivir), which mainly inhibit influenza virus neuraminidase (NA), blocking the release of influenza virus from infected cells (Palese 2004; De Clercq 2006). The second category includes amantadine and rimantadine, which mainly destroy influenza virus M2 protein ion channel activity and inhibit the uncoating process of influenza virus (Jing, Ma et al. 2008). However, US Centers for Disease Control and Prevention has found in sample survey that in the strains of H3N2 in 2008/2009 and pandemic H1N1 virus in 2009, 100% of the strains were resistant to adamantane drugs; 99.6% of the seasonal H1N1 Influenza viruses were resistant to Tamiflu (http://www.cdc.gov/flu/weekly/weeklyarchives2008-2009/weekly35.htm).
Triterpenoids are a class of natural compounds widely found in nature, and its structure includes five rings of A, B, C, D, E and 30 carbon atoms (Hostettmann, K et al. 1995; Waller, G. R. et al. 1996). Triterpenoids caused more and more attention due to various biological and pharmacological activities. For example, betulinic acid and its derivatives have been used as anti-tumor and anti-HIV drugs in clinical trials (U.S. Pat. Nos. 5,679,828; 6,689,767; 6,369,109; U.S. App. Pub. No. 2004/0204389); oleanolic acid is an active ingredient for protecting the liver against chemical damage and controlling HIV infection (Liu, J. et al. 2005); moreover, European researchers recently reported that hawthorn acid can inhibit spread of HIV in the body, with an inhibition rate up to 80%. The prior application 201110373224.3 filed by the patent applicant, which has not been disclosed at present, discloses a class of triterpene derivatives and their use for the prevention and treatment of viral hepatitis, not for the prevention and treatment of influenza. The inhibition of triterpenoids on influenza virus has not been reported.